I think I did understand that when it comes to MKVs since it was popular for anime precisely because of its support for newer x265 codecs. So there is some conn

I think I did understand that when it comes to MKVs since it was popular for anime precisely because of its support for newer x265 codecs. So there is some connection between container and codec, in that not all containers support all codecs. I would also assume since x264 is dated at this point and been around a long time that there would be universal support for those supported codecs but that clearly isn't the case. In any event, it's unexpected that the default configuration of ffmpeg produces an MP4 unplayable on Mac. Looks like "pix_fmt yuv420p" is what's needed. Side note I'm remembering now why I needed to do this, some dev tooling produces WebP video files which can't be uploaded everywhere.